Method and spacer device for spanning a space formed upon removal of an intervertebral disc

ABSTRACT

An intervertebral spacer is designed particularly for patients who are not candidates for total disc replacement. The intervertebral spacer maintains disc height and prevents subsidence with a large vertebral body contacting surface area while substantially reducing recovery time by eliminating the need for bridging bone. The intervertebral spacer or fusion spacer includes a rigid spacer body sized and shaped to fit within an intervertebral space between two vertebral bodies. In one embodiment, the intervertebral spacer body has two opposed metallic vertebral contacting surfaces, at least one fin extending from each of the vertebral contacting surfaces and configured to be positioned within slots cut into the two vertebral bodies. Holes within the vertebral body contacting surfaces to provide increased bone on growth surfaces and to prevent subsidence.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application is a continuation of U.S. patent applicationSer. No. 15/359,298, filed Nov. 22, 2016, which is a continuation ofU.S. patent application Ser. No. 12/255,731, filed Oct. 22, 2008, whichclaims priority from U.S. Provisional Patent Application No. 60/981,665,filed Oct. 22, 2007, the full disclosures of which are incorporatedherein by reference.

BACKGROUND OF THE DISCLOSURE

The present disclosure relates to medical devices and methods. Morespecifically, the disclosure relates to intervertebral spacers andmethods of spanning a space formed upon removal of an intervertebraldisc.

Back pain takes an enormous toll on the health and productivity ofpeople around the world. According to the American Academy of OrthopedicSurgeons, approximately 80 percent of Americans will experience backpain at some time in their life. In the year 2000, approximately 26million visits were made to physicians' offices due to back problems inthe United States. On any one day, it is estimated that 5% of theworking population in America is disabled by back pain.

One common cause of back pain is injury, degeneration and/or dysfunctionof one or more intervertebral discs. Intervertebral discs are the softtissue structures located between each of the thirty-three vertebralbones that make up the vertebral (spinal) column. Essentially, the discsallow the vertebrae to move relative to one another. The vertebralcolumn and discs are vital anatomical structures, in that they form acentral axis that supports the head and torso, allow for movement of theback, and protect the spinal cord, which passes through the vertebrae inproximity to the discs.

Discs often become damaged due to wear and tear or acute injury. Forexample, discs may bulge (herniate), tear, rupture, degenerate or thelike. A bulging disc may press against the spinal cord or a nerveexiting the spinal cord, causing “radicular” pain (pain in one or moreextremities caused by impingement of a nerve root). Degeneration orother damage to a disc may cause a loss of “disc height,” meaning thatthe natural space between two vertebrae decreases. Decreased disc heightmay cause a disc to bulge, facet loads to increase, two vertebrae to rubtogether in an unnatural way and/or increased pressure on certain partsof the vertebrae and/or nerve roots, thus causing pain. In general,chronic and acute damage to intervertebral discs is a common source ofback related pain and loss of mobility.

When one or more damaged intervertebral discs cause a patient pain anddiscomfort, surgery is often required. Traditionally, surgicalprocedures for treating intervertebral discs have involved discectomy(partial or total removal of a disc), with or without interbody fusionof the two vertebrae adjacent to the disc. When the disc is partially orcompletely removed, it is necessary to replace the excised material toprevent direct contact between hard bony surfaces of adjacent vertebrae.Oftentimes, pins, rods, screws, cages and/or the like are insertedbetween the vertebrae to act as support structures to hold the vertebraeand graft material in place while they permanently fuse together.

One typical fusion procedure is achieved by inserting a “cage” thatmaintains the space usually occupied by the disc to prevent thevertebrae from collapsing and impinging the nerve roots. The cage isused in combination with bone graft material (either autograft orallograft) such that the two vertebrae and the graft material will growtogether over time forming bridging bone between the two vertebrae. Thefusion process typically takes 6-12 months after surgery. During in thistime external bracing (orthotics) may be required. External factors suchas smoking, osteoporosis, certain medications, and heavy activity canprolong or even prevent the fusion process. If fusion does not occur,patients may require reoperation.

One known fusion cage is described in U.S. Pat. No. 4,904,261 andincludes a horseshoe shaped body. This type cage is currently availablein PEEK (polyetheretherketone). PEEK is used because it does not distortMRI and CT images of the vertebrae. However, PEEK is a material thatdoes not allow bone to attach. Thus, fusion with a PEEK cage requiresbridging bone to grow through the holes in the cage to providestabilization.

It would be desirable to achieve immobilization of the vertebrae andmaintain spacing between the adjacent vertebrae without the associatedpatient discomfort and long recovery time of traditional interbodyfusion which may require immobilization for several months.

Another problem associated with the typical fusion procedure is thesubsidence of the cage into the vertebral body. The typical fusion cageis formed with a large percentage of open space to allow the bone togrow through and form the bridging bone which immobilizes the discs.However, the large amount of open space means that the load on eachsegment of the cage is significantly higher than if the cage surfacearea was larger. This results in the cage subsiding or sinking into thebone over time causing the disc space to collapse. In addition, the hardcortical bone on the outer surface of the vertebral body that transfersload to the interbody cage or spacer is often scraped, punctured orotherwise damaged to provide blood to the interbody bone graft tofacilitate bone growth. This damage to the bone used to promote bonegrowth can also lead to subsidence.

The U.S. Food and Drug Administration approved the use of a geneticallyengineered protein, or rhBMP-2, for certain types of spine fusionsurgery. RhBMP-2 is a genetically engineered version of a naturallyoccurring protein that helps to stimulate bone growth, marketed byMedtronic Sofamor Danek, Inc. as InFUSE™ Bone Graft. When InFUSE™ isused with the bone graft material it eliminates the need for painfulbone graft harvesting and improves patients' recovery time. However,InFUSE™ adds significantly to the cost of a typical fusion surgery.Additionally, even with the bone graft and InFUSE™ bone may fail to growcompletely between the two vertebrae or the cage may subside into thevertebrae such that the fusion fails to achieve its purpose ofmaintaining disc height and preventing motion.

In an attempt to treat disc related pain without fusion and to maintainmotion, an alternative approach has been developed, in which a movable,implantable, artificial intervertebral disc (or “disc prosthesis”) isinserted between two vertebrae. A number of different artificialintervertebral discs are currently being developed. For example, U.S.Patent Application Publication Nos. 2005/0021146, 2005/0021145, and2006/0025862, which are hereby incorporated by reference in theirentirety, describe artificial intervertebral discs. Other examples ofintervertebral disc prostheses are the LINK SB CHARITLE™ disc prosthesis(provided by DePuy Spine, Inc.) the MOBIDISK™ disc prosthesis (providedby LDR Medical), the BRYAN™ cervical disc prosthesis (provided byMedtronic Sofamor Danek, Inc.), the PRODISC™ disc prosthesis orPRODISC-C™ disc prosthesis (from Synthes Stratec, Inc.), the PCM™ discprosthesis (provided by Cervitech, Inc.), and the MAVERICK™ discprosthesis (provided by Medtronic Sofomor Danek). Although existing discprostheses provide advantages over traditional treatment methods, manypatients are not candidates for an artificial disc due to facetdegeneration, instability, poor bone strength, previous surgery,multi-level disease, and pain sources that are non-discogenic.

Therefore, a need exists for an improved spacer and method for spanninga space and maintaining disc spacing between two vertebrae after removalof an intervertebral disc. Ideally, such improved method and spacerwould avoid the need for growth of bridging bone across theintervertebral space.

BRIEF SUMMARY OF THE DISCLOSURE

Embodiments of the present disclosure provide a rigid intervertebralspacer and methods of spanning a space formed upon removal of anintervertebral disc.

In accordance with one aspect of the present disclosure, a method ofspanning a space formed by upon removal of an intervertebral discincludes the steps of performing a discectomy to remove disc materialbetween two adjacent vertebral bodies; placing an intervertebral spacerbetween the two adjacent vertebral bodies; and maintaining the discspace between the two adjacent vertebral bodies with the intervertebralspacer without the use of bone graft or bridging bone. Theintervertebral spacer includes two end plates, each end plate having ametallic vertebral body contacting surface and an inner surface, and aconnector interconnecting the inner surfaces of the two end plates in arigid manner which limits motion between the end plates to less than atotal of 5 degrees. The vertebral body contacting surfaces of the endplates have no holes therein or have holes which cover less than 40percent of the vertebral body contacting surface.

In accordance with another aspect of the present disclosure, anintervertebral spacer for spanning a space formed by upon removal of anintervertebral disc includes two end plates sized and shaped to fitwithin an intervertebral space and a connector interconnecting the innersurfaces of the two end plates in a rigid manner which limits motionbetween the end plates to less than a total of 5 degrees. Each end platehas a metallic vertebral contacting surface and an inner surface and thevertebral body contacting surfaces of the end plates have no holestherein or have holes which cover less than 40 percent of the vertebralbody contacting surfaces.

In accordance with a further aspect of the disclosure, a method ofperforming an anterior/posterior fusion comprises performing adiscectomy to remove disc material between two adjacent vertebralbodies; placing an intervertebral spacer between the two adjacent discs;maintaining the disc space between the two adjacent discs with theintervertebral spacer; and posteriorly placing a stabilization system tofix the angle between the vertebral bodies. The intervertebral spacerincludes two end plates each having a metallic vertebral contactingsurface and an inner surface, and a rigid connector interconnecting theinner surfaces of the two end plates. The vertebral body contactingsurfaces of the end plates have no holes therein or have holes whichcover less than 40 percent of the vertebral body contacting surfaces.

In accordance with another aspect of the disclosure, a fusion systemincludes an intervertebral spacer and a posteriorly placed stabilizationsystem including at least two screws configured to be placed into thevertebral bodies and at least one connector there between, Theintervertebral spacer includes two end plates sized and shaped to fitwithin an intervertebral space, each end plate having a vertebralcontacting surface an inner surface and a rigid connectorinterconnecting the inner surfaces of the two end plates. The vertebralbody contacting surfaces of the end plates have no holes therein or haveholes which cover less than 40 percent of the vertebral body contactingsurfaces.

In accordance with an additional aspect of the disclosure, a fusionspacer includes a rigid spacer body sized and shaped to fit within anintervertebral space between two vertebral bodies, the body having twoopposed metallic vertebral contacting surfaces; at least one finextending from each of the vertebral contacting surfaces, the finsconfigured to be positioned within slots cut into the two vertebralbodies; and a plurality of serrations on the vertebral contactingsurfaces. Holes, if present, cover less than 40 percent of the entirevertebral body contacting surfaces.

According to further embodiments of the disclosure, a method of spanninga space formed upon removal of an intervertebral disc, the methodincluding: performing a discectomy to remove disc material between twoadjacent vertebral bodies; cutting at least one slot in at least one ofthe adjacent vertebrae; placing an intervertebral spacer between the twoadjacent vertebral bodies, the intervertebral spacer including: two endplates, each end plate having a metallic vertebral body contactingsurface, an inner surface and a fin extending from the vertebral bodycontacting surface; a connector interconnecting the inner surfaces ofthe two end plates in a rigid manner which limits motion between the endplates to less than a total of 5 degrees; wherein the vertebral bodycontacting surfaces of the two end plates have at least one through holetherein that covers less than 40 percent of the vertebral bodycontacting surfaces, and wherein the at least one through hole thereinextends longitudinally from one side of each end plate through the endplate to the other side of the end plate for bone growth therein,wherein the intervertebral spacer including the two end plates andconnector is formed of a single piece; placing a fin on one of thevertebral body contacting surfaces into the at least one slot, wherebythe intervertebral spacer is inhibited from rotating; and maintainingthe disc spaced between the two adjacent vertebral bodies with theintervertebral spacer without the use of bone graft or bridging bone,wherein no part of the intervertebral spacer extends outside theintervertebral disc space and slot.

Additional embodiments of the disclosure provide an intervertebralspacer for spanning a space formed by upon removal of an intervertebraldisc, the intervertebral spacer including: two end plates sized andshaped to fit within an intervertebral space between two vertebrae, eachend plate having a metallic vertebral contacting surface and an innersurface, wherein the vertebral body contacting surfaces of the two endplates have at least one through hole therein that covers less than 40percent of the vertebral body contacting surfaces, and wherein the atleast one through hole therein extends longitudinally from one side ofeach end plate through the end plate to the other side of the end platefor bone growth therein; a connector interconnecting the inner surfacesof the two end plates in a rigid manner which limits motion between theend plates to less than a total of 5 degrees; and at least one finprojecting from one of the vertebral contacting surfaces, wherein thefin is configured to be inserted into a slot cut in the vertebra toinhibit rotation of the intervertebral spacer with respect to thevertebra.

Yet another embodiment of the disclosure provides a method of spanning aspace formed upon removal of an intervertebral disc, the methodincluding: performing a discectomy to remove disc material between twoadjacent vertebral bodies; cutting at least one slot in at least one ofthe adjacent vertebrae; placing an intervertebral spacer between the twoadjacent vertebral bodies, the intervertebral spacer including: two endplates, each end plate having a metallic vertebral body contactingsurface, an inner surface and a fin extending from the vertebral bodycontacting surface; a connector interconnecting the inner surfaces ofthe two end plates in a rigid manner to limits motion between the endplates; wherein the vertebral body contacting surfaces of the two endplates have at least one through hole therein, wherein the at least onethrough hole therein extends longitudinally from one side of each endplate through the end plate to the other side of the end plate for bonegrowth therein; placing a fin on one of the vertebral body contactingsurfaces into the at least one slot, whereby the intervertebral spaceris inhibited from rotating; and maintaining the disc spaced between thetwo adjacent vertebral bodies with the intervertebral spacer without theuse of bone graft or bridging bone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an intervertebral spacer according toone embodiment of the present disclosure;

FIG. 2 is a cross sectional side view of the intervertebral spacer ofFIG. 1;

FIG. 3 is a top view of the intervertebral spacer of FIG. 1;

FIG. 4 is a bottom view of the intervertebral spacer of FIG. 1;

FIG. 5 is a perspective view of an intervertebral spacer according toanother embodiment of the present disclosure;

FIG. 6 is a perspective view of an intervertebral spacer according to anembodiment with added screw fixation; and

FIG. 7 is a perspective view of a further intervertebral spacer withadded screw fixation.

DETAILED DESCRIPTION OF THE DISCLOSURE

Various embodiments of the present disclosure generally provide for anintervertebral spacer having upper and lower plates connected by acentral connector which is substantially rigid. The intervertebralspacer according to the present disclosure can maintain disc height andprevent subsidence with a large vertebral body contacting surface areawhile substantially reducing recovery time by eliminating the need forbridging bone. The fusion spacer described herein is designedparticularly for patients who are not candidates for total discreplacement.

One example of an intervertebral spacer 10 for maintaining disc heightbetween two adjacent vertebral discs is shown in FIG. 1. The spacerincludes two end plates 20, 22, each end plate having a vertebralcontacting surface 24 and an inner surface 26, and a connector 30interconnecting the inner surfaces of the two end plates in asubstantially rigid manner. The intervertebral spacer 10 when implantedbetween two vertebral discs maintains a desirable disc space between thetwo adjacent discs similar to that provided by a natural disc andeliminates the long recovery time required to grow bridging bone whichis required in the traditional fusion surgery.

Although the connector 30 has been shown as circular in cross section,other shapes may be used including oval, elliptical, or rectangular.Although the connector has been shown as a solid member connecting theplates 20, 22 in the center of the plates one or more connectors may beprovided in other configurations and at other locations. By way ofexample, a connector may be the same or substantially the same diameterand shape as the plate, as in FIGS. 6 and 7. Alternatively, multipleconnectors can be arranged in a pattern, such as a rectangular pattern,or a hollow cylindrical connector can be used.

In some embodiments, the outer surface 24 is planar. Oftentimes, theouter surface 24 will include one or more surface features and/ormaterials to enhance attachment of the spacer 10 to vertebral bone. Forexample, as shown in FIG. 2, the outer surface 24 may be machined tohave serrations 40 or other surface features for promoting adhesion ofthe plates 20, 22 to a vertebra. In the embodiments shown, theserrations 40 are pyramid shaped serrations extending in mutuallyorthogonal directions and arranged on opposite sides of a fin 50. Theserrations 40 may also be disposed in a region between fins 52 when theouter surface 24 has two fins. Other geometries such as teeth, grooves,ridges, pins, barbs or the like would also be useful in increasingfixation of the spacer 10 to the adjacent vertebral bodies. When thebone integration structures are ridges, teeth, barbs or similarstructures, they may be angled to ease insertion and prevent migration.These bone integration structures can be used to precisely cut the boneduring implantation to cause bleeding bone and encourage boneintegration. Additionally, the outer surface 24 may be provided with arough microfinish formed by blasting with aluminum oxide microparticlesor the like to improve bone integration. In some embodiments, the outersurface may also be titanium plasma sprayed or HA coated to furtherenhance attachment of the outer surface 24 to vertebral bone.

The outer surface 24 may also carry one or more upstanding fins 50, 52extending in an anterior-posterior direction. The fins 50, 52 areconfigured to be placed in slots cut into the vertebral bodies.Preferably, the fins 50, 52 each have a height greater than a width andhave a length greater than the height. In one embodiment, the fins 50,52 are pierced by transverse holes 54 for bone ingrowth. The transverseholes 54 may be formed in any shape and may extend partially or all theway through the fins 50, 52. In alternative embodiments, the fins 50, 52may be rotated away from the anterior-posterior axis, such as in alateral-lateral orientation, a posterolateral-anterolateral orientation,or the like to accommodate alternate implantation approaches.

The fins 50, 52 provide improved attachment to the bone and preventrotation of the plates 20, 22 in the bone. In some embodiments, the fins50, 52 may extend from the surface 24 at an angle other than 90°. Forexample on one or more of the plates 20, 22 where multiple fins 52 areattached to the surface 24 the fins may be canted away from one anotherwith the bases slightly closer together than their edges at an anglesuch as about 80-88 degrees. The fins 50, 52 may have any other suitableconfiguration including various numbers, angles and curvatures, invarious embodiments. In some embodiments, the fins 50, 52 may be omittedaltogether. The embodiment of FIG. 1 illustrates a combination of afirst plate 20 with a single fin 50 and a second plate 22 with a doublefin 52. This arrangement is useful for double level disc replacementsand utilizes offset slots in the vertebral body to prevent the rareoccurrence of vertebral body splitting by avoiding cuts to the vertebralbody in the same plane for multi-level implants.

The spacer 10 has been shown with the fins 50, 52 as the primaryfixation feature, however, the fins may also be augmented or replacedwith one or more screws extending through the plates and into the bone.For example in the spacer 10 of FIG. 1 the upper fin 50 may be replacedwith a screw while the two lower fins 52 remain. The plates 20, 22 canbe provided with one or a series of holes to allow screws to be insertedat different locations at the option of the surgeon. However, the holesshould not be of such size or number that the coverage of the plate 20,22 is decreased to such an extent that subsidence occurs. When one ormore screws are provided, they may incorporate a locking feature toprevent the screws from backing out. The screws may also be providedwith a bone integration coating.

The upper and lower plates 20, 22 and connector 30 may be constructedfrom any suitable metal, alloy or combination of metals or alloys, suchas but not limited to cobalt chrome alloys, titanium (such as grade 5titanium), titanium based alloys, tantalum, nickel titanium alloys,stainless steel, and/or the like. They may also be formed of ceramics,biologically compatible polymers including PEEK, UHMWPE (ultra highmolecular weight polyethylene) or fiber reinforced polymers. However,the vertebral contacting surfaces 24 are formed of a metal or othermaterial with good bone integration properties. The metallic vertebralbody contacting surfaces 24 may be coated or otherwise covered with themetal for fixation. The plates 20, 22 and the connector 20 may be formedof a one piece construction or may be formed of more than one piece,such as different materials coupled together. When the spacer 10 isformed of multiple materials these materials are fixed together to forma unitary one piece spacer structure without separately moving parts.

Different materials may be used for different parts of the spacer 10 tooptimize imaging characteristics. For example, the plates may be formedof titanium while the connector is formed of cobalt chromium alloy forimproved imaging of the plates. Cobalt chrome molybdenum alloys whenused for the plates 20, 22 may be treated with aluminum oxide blastingfollowed by a titanium plasma spray to improve bone integration. Othermaterials and coatings can also be used such as titanium coated withtitanium nitride, aluminum oxide blasting, HA (hydroxylapatite) coating,micro HA coating, and/or bone integration promoting coatings. Any othersuitable metals or combinations of metals may be used as well as ceramicor polymer materials, and combinations thereof. Any suitable techniquemay be used to couple materials together, such as snap fitting, slipfitting, lamination, interference fitting, use of adhesives, weldingand/or the like.

As shown in FIG. 5, some limited holes 60 may also be provided in theplates 20, 22 to allow bone in growth. Holes provided in a typicalfusion spacer provide a spacer with little structural support andmaximum area for bone growth. Thus, the load transferred across the discspace per unit area of spacer is quite high resulting in possiblesubsidence of the typical spacer. In the spacer 10 of the presentdisclosure, the load transfer is spread across a larger area. If theouter surfaces 24 have holes 60 therein, the holes will cover less than40 percent of the outer surface 24 which contacts the bone to preventsubsidence of the plates into the vertebral bodies. Preferably the holeswill cover less than 25 percent, and more preferably less than 10percent of the outer bone contacting surfaces. At the option of thesurgeon, when the small holes 60 are present in the plates 20, 22, bonegraft can be placed in the space between the inner surfaces 26 of theplates to encourage bone to grow through the plates. The holes 60, whenpresent can take on a variety of shapes including circular, as shown,rectangular, polygonal or other irregular shapes. The holes 60 mayextend through the various parts of the spacer including through theconnector or through the fins. The holes 60 may change shape or size asthey pass through portions of the spacer, for example, holes through theplates and the connector may taper to a smaller interior diameter.

The typical fusion spacer requires bleeding bone to stimulate the growthof bridging bone. In this typical method, the cortical endplates aredamaged purposefully to obtain bleeding by rasping or cutting the bone.This damage weakens the bone and can cause subsidence of the spacer. Thespacer 10 described herein does not rely on bridging bone and does notrequire damaging the bone to cause bleeding. The spacer 10 can beimplanted after simply cleaning the disc space and cutting slots intothe vertebral endplates configured to receive the fins 50, 52. The restof the endplates remain undamaged, providing better support and discheight maintenance.

FIG. 6 shows another embodiment of a spacer 100 having a single fin 50on the top and bottom and two fixation screws 70 extending at an angleof about 30 to about 60 degrees with respect to the vertebral bodycontacting surfaces 24 of the spacer. The spacer 100 also includes aconnector 30 between the vertebral body contacting surfaces 24 which isformed in one piece with the upper and lower plates. The fixation screws70 can include a locking mechanism, such as a locking thread or aseparate locking member which is inserted into the screw holes 80 afterthe screws are inserted to prevent backing out of the screws.

FIG. 7 illustrates an alternative embodiment of a spacer 110 having asingle superior fin 50, two inferior fins 52, and three alternatingholes 80 for receiving bone screws (not shown). The spacer 110 hasmultiple fixation structures to provide the patient near immediatemobility after the fusion procedure. As an alternative to thealternating angled holes 80, the spacer 110 can be formed with ananterior flange extending from the top and the bottom at the anteriorside of the plate. This optional flange can include one or more holesfor receiving bone screws placed laterally. The laterally placed bonescrews can prevent interference in the event of multilevel fusions andare particularly useful for a cervical fusion where space is morelimited.

The intervertebral spacer 10 shown herein is configured for placement ina lumbar intervertebral space from an anterior approach. It should beunderstood that all approaches can be used including PLIF (posteriorlumbar interbody fusion), TLIF (transverse lumbar interbody fusion),XLIF (Lateral extracavitary interbody fusion), ALIF (anterior lumbarinterbody fusion), trans-sacral, and other approaches. The shape of theintervertebral spacer would be modified depending on the approach. Forexample, for a posterior approach, the spacer may include two separatesmaller spacers which are either positioned separately side-by-side inthe intervertebral space or two spacers which are joined together onceinside the intervertebral space. For a lateral approach, theintervertebral spacer may be formed in a more elongated, kidney bean orbanana shape with a transversely oriented fin.

The spacers 10, 100 can be provided in different sizes, with differentplate sizes, angles between plates, lordosis angles, and heights fordifferent patients or applications. The spacers 10, 100 are primarilydesigned for use in the lumbar spine, however the spacers may also beused for fusions of the cervical spine. In one variation, the height ofthe spacer can be adjustable, such as by rotating an adjustment screw inthe connector 30 before or after implantation. The spacers preferablyare sized to provide substantial coverage of the vertebral surfaces. Forexample in an anterior procedure, the plates are sized to cover at least50 percent of the vertebral surface, and preferably cover at least 70percent of the vertebral surface. In posterior or lateral procedures thecoverage of the vertebral surface may be somewhat smaller due to thesmall size of the access area, i.e. the posterior or lateral spacers maycover about 40 percent or more of the vertebral surface with a one ortwo part spacer, and preferably at least 50 percent of the vertebralsurface.

The size of the intervertebral spacers 10, 100, 110 can also bedescribed in terms of the amount of the volume of the intervertebralspace occupied by the spacer. According to a preferred embodiment, thetotal volume of the intervertebral spacer selected for a particularintervertebral space fills at least 50 percent of the volume of thespace available between the adjacent vertebrae. More preferably, thevolume of the spacer is at least 70 percent of the volume of theintervertebral space. The volume of the intervertebral space is definedas the volume of the space between the vertebrae when the vertebrae aredistracted to a normal physiologic position for the particular patientwithout over or under distracting. The size of the intervertebralspacers 10, 100, 110 can also be determined by the amount of the supportprovided to the ring of cortical bone surrounding each vertebrae. Thecortical bone surrounds a more spongy cancellous bone tissue.Preferably, the intervertebral spacer is selected to support at least 75percent of the diameter of the ring of cortical bone.

One common fusion procedure, referred to as an anterior/posteriorfusion, uses of one or more fusion cages to maintain the disc spacewhile bridging bone grows and also uses a system of posterior screws androds for further stabilization. Fusing both the front and back providesa high degree of stability for the spine and a large surface area forthe bone fusion to occur. Also, approaching both sides of the spineoften allows for a more aggressive reduction of motion for patients whohave deformity in the lower back (e.g. isthmic spondylolisthesis).

According to a method of the present disclosure, the anterior approachis performed first by removing the disc material and cutting theanterior longitudinal ligament (which lays on the front of the discspace). The spacer is positioned anteriorly and then the patient isturned over for the implantation of a posterior stabilization system.The intervertebral spacers of the present disclosure may be used incombination with a posterior stabilization system, dynamic rodstabilization system, or interspinous spacer to achieve theanterior/posterior fusion.

In another example, a posterior intervertebral spacer formed in twoparts can be used with a posterior stabilization system including screwsand rods. This system provides the advantage of maintenance of discheight and stabilization with an entirely posterior approach.

While the exemplary embodiments have been described in some detail, byway of example and for clarity of understanding, those of skill in theart will recognize that a variety of modifications, adaptations, andchanges may be employed. Hence, the scope of the present disclosureshould be limited solely by the appended claims.

What is claimed is:
 1. A method of spanning a space formed upon removalof an intervertebral disc, the method comprising: performing adiscectomy to remove disc material between two adjacent vertebralbodies; cutting at least one slot in at least one of the adjacentvertebrae; placing an intervertebral spacer between the two adjacentvertebral bodies, the intervertebral spacer including: two end plates,each end plate having a metallic vertebral body contacting surface, aninner surface, and a fin extending from the vertebral body contactingsurface; a connector interconnecting the inner surfaces of the two endplates in a rigid manner which limits motion between the end plates toless than a total of 5 degrees; wherein the vertebral body contactingsurfaces of one of the two end plates has at least one through holetherein that covers less than 40 percent of the vertebral bodycontacting surfaces, and wherein the at least one through hole thereinextends longitudinally from one side of each end plate through the endplate to the other side of the end plate for bone growth therein,wherein the intervertebral spacer including the two end plates andconnector is formed of a single piece; placing a fin on one of thevertebral body contacting surfaces into the at least one slot, wherebythe intervertebral spacer is inhibited from rotating; and maintainingthe disc spaced between the two adjacent vertebral bodies with theintervertebral spacer without the use of bone graft or bridging bone,wherein no part of the intervertebral spacer extends outside the slot.2. The method of claim 1, wherein the connector is a rigid connector. 3.The method of claim 1, wherein the two end plates and the connectorinclude a metal.
 4. The method of claim 1, wherein the intervertebralspacer fills at least 50 percent of a vertebral space formed between thevertebral bodies when the intervertebral spacer is positioned betweenthe two vertebral bodies.
 5. The method of claim 1, wherein the holes ofthe vertebral body contacting surface of the end plates cover less than25 percent of the vertebral body contacting surfaces.
 6. The method ofclaim 1, further comprising at least one additional coupling componentprovided on the vertebral body contacting surfaces of the intervertebralspacer.
 7. The method of claim 6, wherein the additional couplingcomponent includes at least one of a screw, teeth, serrations orgrooves.
 8. The method of claim 1, wherein the two adjacent vertebralbodies are stabilized without the use of external plates or screws. 9.The method of claim 1, wherein the connector includes a solidcylindrical member.
 10. The method of claim 1, wherein the two endplates and connector are formed of a single piece of PEEK with metallicvertebral body contacting surfaces.
 11. An intervertebral spacer forspanning a space formed by upon removal of an intervertebral disc, theintervertebral spacer comprising: two end plates sized and shaped to fitwithin an intervertebral space between two vertebrae, each end platehaving a vertebral body contacting surface and an inner surface, whereinthe vertebral body contacting surfaces of one of the two end plates hasat least one through hole therein that covers less than 40 percent ofthe vertebral body contacting surfaces, and wherein the at least onethrough hole therein extends longitudinally from one side of each endplate through the end plate to the other side of the end plate for bonegrowth therein; a connector interconnecting the inner surfaces of thetwo end plates in a rigid manner which limits motion between the endplates to less than a total of 5 degrees; and at least one finprojecting from one of the vertebral contacting surfaces, wherein thefin is configured to be inserted into a slot cut in the vertebra toinhibit rotation of the intervertebral spacer with respect to thevertebra.
 12. The intervertebral spacer of claim 11, wherein the two endplates and connector are formed of a single piece of PEEK with metallicscreens or metallic coatings formed directly on the PEEK to provide thevertebral body contacting surfaces.
 13. The intervertebral spacer ofclaim 11, wherein the intervertebral spacer is configured such that whenplaced within the intervertebral space no part of the intervertebralspacer extends outside the intervertebral disc space and slot.
 14. Theintervertebral spacer of claim 11, further comprising at least oneadditional coupling component provided on the vertebral body contactingsurfaces.
 15. The intervertebral spacer of claim 14, wherein theadditional coupling component includes at least one of a screw, teeth,serrations or grooves.
 16. The intervertebral spacer of claim 11,wherein the two end plates and the connector include a metal.
 17. Amethod of spanning a space formed upon removal of an intervertebraldisc, the method comprising: performing a discectomy to remove discmaterial between two adjacent vertebral bodies; cutting at least oneslot in at least one of the adjacent vertebrae; placing anintervertebral spacer between the two adjacent vertebral bodies, theintervertebral spacer including: two end plates, each end plate having ametallic vertebral body contacting surface, an inner surface and a finextending from the vertebral body contacting surface; a connectorinterconnecting the inner surfaces of the two end plates in a rigidmanner to limits motion between the end plates; wherein the vertebralbody contacting surfaces of one of the two end plates has at least onethrough hole therein, wherein the at least one through hole thereinextends longitudinally from one side of each end plate through the endplate to the other side of the end plate for bone growth therein;placing a fin on one of the vertebral body contacting surfaces into theat least one slot, whereby the intervertebral spacer is inhibited fromrotating; and maintaining the disc spaced between the two adjacentvertebral bodies with the intervertebral spacer without the use of bonegraft or bridging bone.
 18. The method of claim 17, further comprisingat least one additional coupling component provided on the vertebralbody contacting surfaces of the intervertebral spacer.
 19. The method ofclaim 17, wherein the connector includes a solid cylindrical member. 20.The method of claim 17, wherein the two end plates and connector areformed of a single piece of PEEK with metallic vertebral body contactingsurfaces.